1. Field of the Invention
The present invention relates to a power supply. More particularly, the present invention relates to a control circuit with protection circuit for power supply.
2. Description of the Prior Art
As the technology is improving and more and more electronic products are developed for people's need with better functions which make people's life convenient. Most of the electronic products need a power supply which receives the AC input voltage and providing suitable power for electronic products with safety.
Referring to the FIG. 1, it shows a circuit diagram of a conventional power supply. As shown in FIG. 1, the conventional power supply comprises a transformer T1 having a primary winding NP and a secondary winding NS. One terminal of the primary winding NP is coupled to an output terminal of a rectifier 10. Another terminal of the primary winding NP is coupled to a power transistor Q1. The power transistor Q1 is coupled to the ground through a sense resistor RS. The sense resistor RS senses a switching current IP of the power transistor Q1 for generating a sense signal VCS. The rectifier 10 rectifies an AC (alternative current) input voltage VAC. The output terminal of the rectifier 10 is coupled to a filter capacitor Cbulk to filter the output of the rectifier 10 for generating a rectifying voltage Vbulk at the filter capacitor Cbulk. One terminal of the secondary winding NS of the transformer T1 is coupled to one terminal of a rectifier DO. An output capacitor CO is coupled between another terminal of the rectifier DO and another terminal of the secondary winding NS. The output capacitor CO is also coupled to an output terminal of the power supply which provides a regulated output voltage VO.
As shown in FIG. 1, the control circuit of the conventional power supply comprises a flip-flop 20 to generate a switching signal VPWM for controlling the power transistor Q1 of the power supply. A clock input terminal CK of the flip-flop 20 is coupled to an oscillator 22 to receive a pulse signal PLS generated by the oscillator 22. An input terminal D of the flip-flop 20 is coupled to a supply voltage VCC. An output terminal Q of the flip-flop 20 generates the switching signal VPWM for controlling the power transistor Q1 of the power supply. The control circuit of the power supply further comprises a voltage-divider having two resistors RA and RB. The resistor RA is coupled to the output terminal of the rectifier 10. The resistor RB is coupled between the resistor RA and the ground. A filter capacitor CF is coupled to the resistor RB in parallel for filtering and generating an input voltage VIN.
As shown in FIG. 1, a positive input terminal of a comparator 31 receives a threshold signal VM. A negative input terminal of the comparator 31 is coupled to the sense resistor RS to receive the sense signal VCS for comparing the threshold signal VM with the sense signal VCS. A positive input terminal of a comparator 32 receives the input voltage VIN. A negative input terminal of the comparator 32 receives a reference voltage VREF for comparing the input voltage VIN with the reference voltage VREF. An AND gate 33 receives the outputs of the comparators 31 and 32 to generate a reset signal RST which turns off the switching signal VPWM for protecting the power supply.
Referring to the FIG. 2, it shows the waveforms of the rectifying voltage Vbulk of the filter capacitor Cbulk and the input voltage VIN of the filter capacitor CF. The input voltage VIN is generated by the rectifying voltage Vbulk through the voltage-divider and further filtered by the filter capacitor CF. The positive input terminal of the comparator 32 receives the input voltage VIN. As shown in FIG. 2, the filter capacitor CF filters the input voltage VIN which is divided by the rectifying voltage Vbulk, but the input voltage VIN still has the voltage ripple. The voltage ripple causes that the comparator 32 can not precisely compare the input voltage VIN with the reference voltage VREF. Therefore, the output of the comparator 32 can not control the switching signal VPWM to reduce an output of the power supply accurately. It means that the control circuit of the power supply is not protected accurately, and therefore the safety of the power supply is reduced.
Thus, according to the problems described above, the present invention provides a control circuit with a protection circuit for power supply that can control switching signal for protecting the power supply in response to the peak of the AC input voltage accurately.